1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to direct conversion radio frequency receivers.
2. Description of the Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one intermediate frequency stage for direct conversion, and a power amplifier. The data modulation stage converts raw data into complex baseband signals (i.e., in-phase signal components and quadrature signal components) in accordance with a particular wireless communication standard. The intermediate frequency stage mixes the complex baseband signals with a local oscillation to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one intermediate frequency stage for direct conversion, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The intermediate frequency stage mix the amplified RF signals with a local oscillation having an in-phase component and a quadrature component to convert the amplified RF signal into complex baseband signals. The filtering stage filters the complex baseband signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
Ideally, the complex baseband signals produced by the direct conversion receiver are orthogonal (i.e., signals of the same magnitude but phase shifted by 90 degrees). However, due to component limitations of the receiver, ideal complex signals are not achieved. The actual complex signals include an I-Q mismatch, where the magnitudes of the I and Q signal components may not be identical and/or may be shifted by more or less than 90 degrees. Such I-Q mismatches can cause errors in the recapturing of data.
Therefore, a need exists for a method and apparatus for minimizing I-Q mismatches in direct conversion receivers.